Add explanation pop-ups

[lauraferrarese]

Let's try the following for a start: next to "Query Catalogues" add a "?", which , when clicked, opens a bubble with explanations about the field. An example for the text in the bubble is below (but feel free to edit if you don't think this is clear enough or if info is missing). IT WOULD BE VERY USEFUL IF IN THE CODE THE TEXT WERE TO BE SOMEWHERE WHICH CAN BE EASILY ACCESSED/EDITED, in case we want to make changes down the line.....

For an example of the format, see: http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/search/?collection=CFHT&noexec=true. If this works, we an then add more explanations for other fields.

"The following catalogues can be displayed, queried and downloaded:

cfht.ngvsCatalog: all certain, likely and probable galaxies belonging to the Virgo cluster
cfht.ngvsGCXDCatalog: Globular Clusters belonging to Virgo
cfht.ngvsStarsXDCatalog: foreground stars

To load a catalogue, click "Apply". After a catalogue has been loaded, checking (unchecking) the box on the right-end selection field toggles the display on (off), but does not remove a catalogue from memory.
To download a csv table of the catalogue selected, click "Download".
To remove a catalogue from memory, click "Clear".

Catalogue searches can use a single value, multiple discrete value, or a range of values. Values within a range should be specified by the range boundaries separated by two dots, while discrete values should be separated by commas. For example, entering 12.2,12.57,12.6 under "RA" selects all objects with RA exactly equal to 12.2, 12.57 and 12.6. Entering 12.2..12.6 selects all objects with RA between 12.2 and 12.6." If a search has been performed on a catalogue, clicking "Download" downloads the selection, rather than the entire catalogue."